1. Field of the Invention
The invention relates to a temperature controlled exhaust assembly for a semiconductor wafer processing system and, more particularly, to an integrated exhaust assembly having temperature control with cold trap capability.
2. Description of the Background Art
In the development of wafer processing equipment for device manufacture, the design of an exhaust system may be as important as that for the process chamber. Many, if not most, of the processes used in semiconductor device fabrication involve either corrosive or toxic chemical precursors. Very often, the process reactions lead to similarly toxic by-products, or leave undesirable deposits on interior surfaces of the chamber and/or exhaust assembly. Therefore, the design of an exhaust system should address environmental concerns and safety considerations for operating personnel, as well as the need for ease of maintenance of system components.
Different types of traps are commercially available for use in an exhaust line to trap a variety of chemicals. Examples include molecular sieve traps which work by chemisorption and cold temperature traps for trapping condensable materials. Off-the-shelf cold traps typically involve only a single stage design, and may not have sufficient trapping efficiency to meet certain processing demands. One example of a process that exceeds the capabilities of existing cold traps is the deposition of titanium nitride (TiN) film from a reaction between titanium tetrachloride (TiCl4) and ammonia (NH3). In addition to the reaction products titanium nitride (TiN), nitrogen (N2) and hydrogen chloride (HCl), other by-products such as adduct ammonia salts are formed. It is found that existing single-stage cold traps cannot effectively trap reaction by-products under certain operating and pumping conditions, resulting in the need for additional design remedies.
Another level of complexity also arises because the nature of the material deposit from the TiCl4/NH3 reaction is temperature dependent Therefore, TiN film deposition is often performed at a temperature of preferably above 600xc2x0 C. In designing a TiN deposition chamber using a high temperature reaction, it is also desirable to maintain the exterior chamber walls at a lower temperature to ensure the safety of operating personnel. Such a high temperature chemical vapor deposition chamber for TiN film deposition is described in a commonly-assigned U.S. Pat. No. 6,364,954, entitled xe2x80x9cHigh Temperature Chemical Vapor Deposition Chamberxe2x80x9d, issued on Apr. 2, 2002, and is herein incorporated by reference. This high temperature chamber comprises a heated liner which is thermally isolated from the chamber body such that the chamber exterior remains at a temperature of about 60xc2x0 C. Since TiN film or reaction by-products are also deposited on the interior surfaces of the chamber, periodic cleaning is needed to maintain reliable process performance. It is known in the art that for the TiCl4/NH3 based chemistry, a small amount of TiN film is formed at a temperature between 150xc2x0 C. to 250xc2x0 C. This film can readily be removed by a chlorine-based chamber cleaning process. Below 150xc2x0 C., however, an adduct salt powder deposit is formed, but it is resistant to the chlorine-based cleaning process. It is thus highly desirable to maintain the interior walls of the chamber and exhaust assembly at a temperature between 150xc2x0 C. to 250xc2x0 C. to facilitate routine chamber cleaning and system maintenance.
Therefore, a need exists in the art for a temperature controlled exhaust system.
The present invention is a temperature-controlled exhaust assembly with cold trap capability, which can be used in conjunction with a variety of process chambers for different semiconductor wafer processing applications. Specifically, the inventive exhaust assembly contains a conduit for exhaust gases that has at least one attached heater and temperature sensor, and a cold trap connected to the conduit. By maintaining the temperature of the conduit within a range appropriate for the specific process, deposit formation on the interior walls of the conduit can be controlled to reduce the frequency of routine chamber maintenance. A water-cooled cold trap is also provided to adsorb any condensables from the exhaust gases prior to their reaching an exhaust pumping system. The present invention has been used in conjunction with a high temperature chemical vapor deposition chamber for TiN film deposition using a TiCl4/NH3 chemistry. Since the nature and property of the deposit from this reaction is temperature dependent, it is important to carefully control the chamber temperature in order to achieve optimal process and chamber performance. For example, chamber maintenance is greatly facilitated by maintaining the exhaust assembly within a temperature range of about 150-250xc2x0 C., because the TiN deposit formed on the interior surfaces of the exhaust assembly can readily be removed by a chlorine-based chamber cleaning process. A cold trap is also provided to adsorb condensables, such as hydrogen chloride (HCl), or other reaction by-products from the exhaust gases.
One embodiment of the invention comprises a multi-heater design, which incorporates a total of six external heaters to heat different portions of the exhaust assembly. Each heater also has an associated temperature sensor to allow for independent closed-loop feedback control by a controller. Furthermore, this embodiment incorporates a single stage multi-loop coil cold trap as an integral component of the exhaust assembly. The cold trap comprises a single baffle plate located close to the entrance of the trap, and a multi-loop cooling coil carrying cooling water at a temperature of about 20-25xc2x0 C. Condensables from the exhaust gases are adsorbed onto surfaces of the baffle plate and coil. When used in conjunction with the TiN deposition chamber using TiCl4/NH3 chemistry, the entire exhaust assembly is maintained within the same temperature range of 150-200xc2x0 C. While this particular application does not take full advantage of the multi-heater design by using a variety of temperatures, it is recognized that this embodiment can potentially provide process control flexibility through independent multi-zone temperature control.
Another embodiment comprises a compact integrated multi-valve uni-body assembly with a single heater that controls the temperature of the assembly. Several valves are mounted onto a single aluminum body which also accommodates a thermoelectric heater and a temperature sensor. For applications requiring temperature control within a single range, this single-heater design greatly simplifies the closed-loop temperature control operation. The unique compact multi-valve uni-body design allows for easy control within a temperature range of about 150-200xc2x0 C. when used in conjunction with the high temperature TiN deposition chamber.
The valve body also provides vacuum ports for adapting to a removable multi-stage trap assembly, which incorporates a multi-stage cold trap and a particle trap. The cold trap provides multi-stage adsorption through a series of cold baffle plates. The baffle plates contain an arrangement of apertures that are offset from each other. As such, the probability of collisions between gas molecules and the baffle plate surfaces is increased, leading to significant improvement in trapping efficiency for condensables such as HCl, or other reaction by-products.
This embodiment provides added operational flexibility because the multi-stage trap can readily be isolated and removed from the rest of the exhaust assembly. By closing two compact shut-off valves provided respectively at the inlet and outlet of the trap, the adsorbed condensables can be safely contained within the trap and transported to other locations for proper disposal. With its flexible temperature control and compact design, the exhaust and trap assembly can readily be retrofitted and adapted for use with any vacuum chamber for a variety of process applications.